Championship leader Lewis Hamilton says Ferrari will be tough to beat for the rest of the season.
Ferrari expected to struggle relatively in Singapore, having finished a minute behind Hamilton at the Hungarian Grand Prix in early August, a similarly tight and twisty track to Marina Bay.
But an upgrade to the Ferrari in Singapore made it competitive and Leclerc was able to take pole, from Hamilton and Vettel.
Hamilton said: “I’m sure we do have upgrades coming at some stage. I don’t know how much is coming, but these next few races are going to be tough.”
Hamilton remains in a commanding position in the world championship, leading team-mate Valtteri Bottas by 65 points, with Leclerc and Verstappen 96 behind and Vettel a further six adrift.
But Hamilton said Mercedes still had work to do to seal their sixth consecutive world title, which would be an all-time record.
Hamilton’s championship position was founded on a run of seven victories in the first 10 races of the season, but he has won only once in the last five grands prix.
“It hasn’t really changed much for me; it’s just as hard,” he said.
“You won’t realise how hard we work in the background. Even the races we’ve won have not come as easily as you probably perceived it.
“There is so much work in the background, to get the starts, to get the strategy right, to get the car in the (set-up) window.
“Of course it’s got tighter and tighter as the season has gone on. But at the same time we just have to be doing a better job. We were doing such a great job at the beginning of the year.
“We have this gap. But the gap could easily go. It is not won yet. We have to pull our socks up. We can squeeze more out of this car and team.
“Together, we can do better. We win and lose as a team and we all feel not great right now and that’s a good thing, because if anyone in the team is feeling relaxed right now, they need a talking to.
“We go to the next race and try to rev ourselves up and do a better job.”
F1 reconvenes this weekend at the Russian Grand Prix at Sochi on the Black Sea coast.
It is a track where Bottas has always been strong, but Mercedes’ concern is that the track has a series of long straights where Ferrari’s more powerful engine will give them an advantage, just as in Belgium and Italy.
There is also a long run from the start to the first corner, so even if Mercedes qualify ahead of the Ferraris, there is a strong possibility of the Italian cars slipstreaming past and into the lead.